Synthetic method for preparing small palladium nanocubes

ABSTRACT

The present disclosure relates to a method for synthesizing Pd nanocubes having an average size less than 10 nm. The reaction temperature, reaction time, and molar ratios of TOP/Pd-OLA can be used to control size and formation of the Pd nanocubes. The present disclosure is also directed to Pd nanocubes, less than 10 nm, having face centered cubic structures. Pd nanocubes of the present disclosure are an effective catalyst for CO 2  reduction reaction with excellent selectivity for CO. Small sized Pd nanocubes can be used not only as the seeds to prepare other metal nanocubes, but can also as powerful catalysts for a wide variety of reactions in different industrial processes.

TECHNICAL FIELD

The present disclosure is directed to a method for preparation of metalnanostructures and metal nanocatalysts, such as palladium nanocubes.

BACKGROUND

Metal nanostructured materials have received considerable interest inindustrial processes, catalytic processes, synthetic chemistry, andcommercial devices. For example, palladium (Pd) nanostructures areimportant catalysts in carbon-carbon coupling reactions, CO₂ reduction,and hydrogenation reactions due to the large diversity of species, arich combination of different oxidation states and ligand coordinationmodes. Carbon dioxide reduction has gained considerable attention as arenewable resource and as a way to produce various chemical productsusing different catalysts. Using metal nanostructures as a catalyst forcarbon dioxide reduction can affect the thermodynamics and kinetics ofthe CO₂ reduction reaction, while the surface features or facets of themetal nanostructures can affect various aspects of the catalyticprocess, including which products are favored. The structure of variousmetal nanostructures and nanocubes can vary depending upon the methodsand conditions used for production. Accordingly, synthetic methods forproducing metal nanostructures, such as polyhedrons and nanocubes, havegarnered attention because of the potential ability to tailor subsequentcatalytic reactions using pre-designed features of the nanostructures,such as surface facets, surface area, and nanostructure size.Predictable, tunable, and scalable synthetic methods are needed toproduce metal nanostructures, specifically at the smaller nanometerscale. Thus, there is a need to synthesize metal cubic or polyhedralnanocatalysts for a rich variety of organic reactions.

SUMMARY

The following presents a simplified summary of one or more aspects ofthe present disclosure in order to provide a basic understanding of suchaspects. This summary is not an extensive overview of all contemplatedaspects and is intended to neither identify key or critical elements ofall aspects nor delineate the scope of any or all aspects. Its purposeis to present some concepts of one or more aspects in a simplified formas a prelude to the more detailed description that is presented later.

The present disclosure is directed to a method for synthesizing metalnanostructures with certain index facets and certain sizes, the methodusing one or more trialkylphosphines as a shape control ligand in thereaction mixture. In some embodiments, the metal nanostructures cancomprise palladium nanostructures and the shape control ligand cancomprise trioctylphosphine (TOP). Palladium nanostructures withcontrolled facets demonstrate superior catalytic performance for oxygenreduction reactions, carbon dioxide reduction reactions, and variousother chemical reactions. Theoretical studies find that surface facetsand terraces are more catalytically active and selective for C—O thanflat palladium. Palladium nanocubes with six facets are regarded as oneof the most active catalysts for carbon dioxide reduction. The presentlydisclosed method can surprisingly control the size of palladiumnanocubes below 10 nm, for example, by controlling the molar ratio ofTOP to palladium, by controlling the reaction time, by controlling thereaction temperature, or combinations of the reaction conditionsthereof.

Moreover, the palladium nanocubes produced by the method disclosedherein demonstrate superior activity and selectivity for carbon dioxidereduction reactions. This disclosure is also directed to metal nanocubesand nanostructures provided by the method described herein and devicescomprising the metal nanocubes and nanostructures provided by the methoddescribed herein, as well as methods of using the same. This disclosureis not limited to palladium metal as the methods described herein can beapplied to other metals, for example, gold, silver, and platinum. Theseand other aspects of the invention will become more fully understoodupon a review of the detailed description, which follows.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a TEM image of 3.1 nm Pd nanocubes synthesized at a molarratio of 10:1 of TOP/Pd-OLA for a reaction time of 20 min according tosome aspects of the present disclosure.

FIG. 2 shows a TEM image of 5.8 nm Pd nanocubes synthesized at a molarratio of 10:1 of TOP/Pd-OLA for a reaction time of 60 min according tosome aspects of the present disclosure.

FIGS. 3A-3D show TEM images of Pd nanocubes synthesized at differentmolar ratios of TOP/Pd-OLA for a reaction time of 20 min, TOP/Pd-OLAmolar ratio of 15:1 (FIG. 3A), TOP/Pd-OLA molar ratio of 5:1 (FIG. 3B),TOP/Pd-OLA molar ratio of 2:1 (FIG. 3C), and no TOP, a molar ratio ofzero (FIG. 3D) according to some aspects of the present disclosure.

FIG. 4A shows an XRD pattern of irregular Pd nanoparticles with theaverage size of 1.9 nm.

FIG. 4B shows an XRD pattern of Pd nanocubes with average size of 3.1 nmaccording to some aspects of the present disclosure.

FIG. 4C shows an XRD pattern of Pd nanocubes with average size of 5.7 nmaccording to some aspects of the present disclosure.

FIG. 5 shows electrocatalytic activity of 3.1 nm Pd nanocubes forelectrochemical reduction of CO₂ at different potential vs RHE accordingto some aspects of the present disclosure.

DETAILED DESCRIPTION

The detailed description set forth below in connection with the appendeddrawings is intended as a description of various configurations and isnot intended to represent the only configurations in which the conceptsdescribed herein may be practiced. The detailed description includesspecific details for the purpose of providing a thorough understandingof various concepts. However, it will be apparent to those skilled inthe art that these concepts may be practiced without these specificdetails. In some instances, well known components are shown in blockdiagram form in order to avoid obscuring such concepts.

Various aspects of this disclosure relates to a method of synthesizingmetal nanocubes, where the edges of the nanocubes have an average lengthof about or less than 10 nanometers. The method described in the currentdisclosure can significantly and precisely control the average particlesize and optionally produces Pd nanocubes having an average edge lengthabout or less than 7 nm.

According to some aspects, the present disclosure is directed to amethod for preparing metal nanocubes and metal nanostructures. In someembodiments, the method can comprise preparation of a metal complexsolution. According to some aspects, the metal complex solution iscombined with a hot reaction mixture under an inert atmosphere, forexample, by hot-injection. The hot reaction mixture contains a shapecontrol ligand. The metal nanostructures subsequently form in the hotreaction mixture. If the metal complex solution comprises palladium andthe hot reaction mixture comprises trioctylphosphine (TOP) as a shapecontrol ligand, the palladium nanostructures will comprise uniformpalladium nanocubes (FIG. 1). The method is according to some aspects,wherein preparing a reaction mixture further comprises preparing areaction mixture comprising a second complexing agent, for example,oleylamine (OLA), and a shape control ligand by heating the reactionmixture under inert atmosphere.

The palladium complex solution may comprise one or more palladiumcomplexes. As used herein, the term “palladium complex” refers to acomplex of palladium and one or more complexing agents. Complexingagents useful according to the present disclosure include, but are notlimited to, tetradecylamine (TDA), dodecylamine (DDA), hexadecylamine(HDA), octadecylamine (ODA), and oleylamine (OLA). According to someaspects, the palladium complex may be provided by combining one or morepalladium atoms or palladium salts with one or more complexing agents ina solution under an inert atmosphere and stirring or sonicating for anacceptable length of time at an acceptable temperature. For example, thepalladium complex may be provided by combining a palladium salt and oneor more complexing agents in a solution under an inert gas flow.Non-limiting examples of other metals that can be substituted forpalladium in the methods herein are gold, silver, and platinum. Examplesof inert gases include, but are not limited to, nitrogen gas, argon gas,helium gas, radon gas, neon gas, xenon gas, and combinations thereof.The combined solution may then be mixed or sonicated at ambienttemperature for about one minute to about one hour, or optionally about20 minutes to 40 minutes, to provide a palladium complex solutioncomprising the palladium complex.

According to some aspects, the palladium nanostructures may be providedby heating the palladium complex solution with a shape control ligand.For example, the palladium nanostructures may be provided by combiningthe palladium complex solution with one or more shape control ligands ata combining temperature under an inert atmosphere for an acceptablelength of time. For example, the palladium nanostructures may beprovided by combining the palladium complex solution with a shapecontrol ligand under an inert atmosphere at an elevated temperature ofbetween about 100 and 500° C., optionally between about 150 and 350° C.,optionally between about 175 and 250° C., and optionally about 200° C.The combined solution may be held at the combining temperature for acombining time of between about 1 minute and 2 hours, optionally betweenabout 1 minute and 1 hour, optionally between about 1 minute and 30minutes, or optionally between about 1 minute and 20 minutes; then, thetemperature may optionally be increased to a reaction temperature. Thereaction temperature may be between about 100 and 500° C., optionallybetween about 150 and 350° C., optionally between about 175 and 300° C.,and optionally about 250° C. The combined solution may be held at thereaction temperature for a reaction time of between about 1 minute and 2hours, optionally between about 1 minute and 1 hour, optionally betweenabout 1 minute and 30 minutes, or optionally between about 1 minute and20 minutes, to provide a palladium nanostructure solution containing thepalladium nanostructures. Examples of shape control ligands include, butare not limited to tributylphosphine, tributylphosphine oxide,trioctylphosphine, trioctylphosphine oxide, oleylamine, tetradecylamine,dodecylamine, octadecylamine, hexadecylamine, oleic acid, andcombinations thereof.

According to some aspects, the palladium complex solution comprisespalladium and oleylamine (OLA). Further according to some aspects, theshape control ligand is trioctylphosphine (TOP). In some embodiments,the molar ratio of TOP/Pd after combining the palladium complex and TOPin the reaction mixture is between about 50:1 and 1:10, optionallybetween about 50:1 and 1:1, optionally between about 20:1 and 1:2,optionally between about 20:1 and 1:1, and optionally between about 10:1and 1:1. In an embodiment, no TOP is in the reaction mixture, the molarratio of TOP/Pd is zero, and aggregated palladium nanoparticles areformed (FIG. 3D).

In some embodiments, the combining temperature is 200° C., the combiningtime is 20 minutes, the molar ratio of TOP/Pd is 10:1, the reactiontemperature is 250° C., the reaction time is 20 minutes, and Pdnanocubes of average size 3.1 nm are formed (FIG. 1). In someembodiments, the combining temperature is 200° C., the combining time is20 minutes, the molar ratio of TOP/Pd is 10:1, the reaction temperatureis 250° C., the reaction time is 60 minutes, and Pd nanocubes of averagesize 5.8 nm are formed (FIG. 2).

In some embodiments, the combining temperature is 200° C., the combiningtime is 20 minutes, the molar ratio of TOP/Pd is 15:1, the reactiontemperature is 250° C., the reaction time is 20 minutes, and Pdnanocubes of average size 5.7 nm are formed (FIG. 3A). In someembodiments, the combining temperature is 200° C., the combining time is20 minutes, the molar ratio of TOP/Pd is 5:1, the reaction temperatureis 250° C., the reaction time is 20 minutes, and Pd nanocubes of averagesize 4.3 nm are formed (FIG. 3B). In some embodiments, the combining thecombining temperature is 200° C., the combining time is 20 minutes, themolar ratio of TOP/Pd is 5:1, the reaction temperature is 250° C., thereaction time is 20 minutes, and Pd nanostructures of average size 1.9nm are formed (FIG. 3C). According to some aspects, no TOP is in thereaction mixture, the combining temperature is 200° C., the combiningtime is 20 minutes, the molar ratio of TOP/Pd is zero, the reactiontemperature is 250° C., the reaction time is 20 minutes, and aggregatedPd nanoparticles are formed (FIG. 3D).

According to some aspects, a palladium complex solution can be preparedby combining a palladium salt, for example, palladium acetylacetonate,with OLA, at a Pd concentration of 0.1 millimolar, and sonicating forabout 30 minutes. The Pd concentration can be fraom about 0.05millimolar to about 2 millimolar, optionally about 0.05 millimolar toabout 1 millimolar, optionally about 0.05 millimolar to about 0.5millimolar, and optionally about 0.1 millimolar. Examples of palladiumsources include, but are not limited to, palladium acetylacetonate,palladium acetate, palladium nitrate, palladiumhexafluoroacetylacetonate, and palladium trifluoroacetate.

According to some aspects, the metal nanostructures are isolated byinjecting or adding ethanol, or another hydrophilic solvent such asmethanol or acetone, to the reaction mixture, centrifuging, anddiscarding the supernatant, retaining the nanostructures in thesediment. The nanostructures can be washed one or more times, forexample, by adding a mixture of hexane and ethanol to the sediment,centrifuging, and discarding the supernatant. The metal nanostructurescan be stored in hydrophobic solvents (e.g. hexane, toluent, andchloroform) before use or characterization, depending on theapplication.

According to some aspects, a method for preparing metal nanocubes isdisclosed herein, the method comprising: providing a metal complexsolution comprising a metal and a first complexing agent; preparing areaction mixture comprising a shape control ligand by heating thereaction mixture under inert atmosphere to a combining temperature;combining the metal complex solution and the reaction mixture at thecombining temperature under inert atmosphere; holding the reactionmixture at the combining temperature for a combining time under inertatmosphere; heating the reaction mixture to a reaction temperature underinert atmosphere; and holding the reaction mixture for a reaction timeat the reaction temperature under inert atmosphere to form metalnanocubes. The method may further comprise cooling the reaction mixturecontaining the metal nanocubes and isolating the metal nanocubes fromthe reaction mixture. The metal utilized can comprise, by way ofnon-limiting example, palladium, gold, silver, and platinum. The shapecontrol ligand can comprise one or more trialkylphosphines, for example,tributylphosphine, tributylphosphine oxide, trioctylphosphine,trioctylphosphine oxide, oleylamine, tetradecylamine, dodecylamine,octadecylamine, hexadecylamine, oleic acid, and combinations thereof.

In some embodiments, the metal complex solution is provided by mixingthe metal, suitable salt form of the metal, or other suitable form, withan alkylamine to form the metal complex solution, wherein the alkylamineis chosen from oleylamine, tetradecylamine, hexadecylamine,octadecylamine, and combinations thereof. According to some aspects, thecombining temperature is 185° C. to 220° C. and the reaction temperatureis 240° C. to 260° C. In some embodiments, the combining time is 15 to25 minutes and the reaction time is 15 to 25 minutes.

For example, the metal complex solution can be provided by mixing ametal salt comprising palladium acetylacetonate with oleylamine until asolution comprising palladium is formed, or other metals, metal salts,or metal forms can be utilized to form a metal complex solution.According to some aspects, palladium metal is utilized, the shapecontrol ligand is trioctylphosphine, and the molar ratio oftrioctylphosphine to palladium in the reaction mixture is from 15:1 to1:1. In some embodiments, the combining temperature is 200° C., thecombining time is 20 minutes, the reaction temperature is 250° C., thereaction time is 20 minutes, the molar ratio of trioctylphosphine topalladium in the reaction mixture is 10:1, and the metal nanocubescomprise palladium nanocubes comprising an average size of 3.1 nm.

In some embodiments, the combining temperature is 200° C., the combiningtime is 20 minutes, the reaction temperature is 250° C., the reactiontime is 60 minutes, the molar ratio of trioctylphosphine to palladium inthe reaction mixture is 10:1, and the metal nanocubes comprise palladiumnanocubes comprising an average size of 5.8 nm. In some embodiments, thecombining temperature is 200° C., the combining time is 20 minutes, thereaction temperature is 250° C., the reaction time is 20 minutes, themolar ratio of trioctylphosphine to palladium in the reaction mixture is15:1, and the metal nanocubes comprise palladium nanocubes comprising anaverage size of 5.7 nm. In some embodiments, the combining temperatureis 200° C., the combining time is 20 minutes, the reaction temperatureis 250° C., the reaction time is 20 minutes, the molar ratio oftrioctylphosphine to palladium in the reaction mixture is 5:1, and themetal nanocubes comprise palladium nanocubes comprising an average sizeof 4.3 nm.

The methods disclosed herein may further comprise washing the metalnanocubes with a second organic solvent or by any means known in theart. Isolation of the metal nanocubes can be, for example, bycentrifugation or by other means. In some embodiments, a plurality ofpalladium nanocubes comprising an average size from 1.9 nm to 5.8 nm, aplurality of palladium nanocubes comprising an average size of 3.1nanometers, a plurality of palladium nanocubes comprising an averagesize of 5.8 nanometers, and a plurality of palladium nanocubescomprising an average size of 4.3 nanometers is disclosed herein.According to some aspects, a catalyst for CO2 reduction comprising thepalladium nanocubes of average size 3.1 nanometers is provided herein, amethod for CO2 reduction comprising the catalysts disclosed herein, andthe palladium nanocubes of average size less than 10 nanometers aredisclosed herein, wherein the palladium nanocubes have a reactionselectivity for carbon monoxide with a FE of 56% at −1.0 V vs. RHE.

Acording to some aspects, the method may comprise a one-step syntheticstrategy. As used herein, the term “one-step synthetic strategy” refersto a synthetic strategy wherein at least a first reactant is convertedto a reaction product in a single synthesis step. For example, asdescribed herein, the palladium complex solution may be converted topalladium nanocubes in a single synthesis step, in particular, providedthe shape control ligand and reaction conditions herein.

Various aspects of this disclosure relates to Pd nanocubes synthesizedusing the present method. Small Pd nanocubes had been obtained inhydrophobic phase by hot-injection method at 250° C. TEM resultsindicated that the reaction temperature and molar ratios of TOP/Pd-OLAplayed critical roles during the formation of Pd nanocubes. The averagesize of Pd nanocubes may be controlled from about 2 nm to about 7 nmusing the present synthesis method. XRD results showed that Pd nanocubeshave face centered cubic structures (FIGS. 4A-4C). Pd nanocubes producedaccording to the methods disclosed herein acting as catalysts for CO₂reduction reaction showed excellent selectivity for CO, which can reachthe maximum FE of 56% at −1.0 V (FIG. 5). The small sized Pd nanocubesmay not only be used as the seeds to prepare other metal nanocubes, butcan also act as powerful catalysts for a wide variety of reactions indifferent industrial processes.

Trioctylphosphine molar concentration ratio compared to the metal molarconcentration ratio, reaction time, and the reaction temperature weredemonstrated to play a very important role for the formation of Pdnanocubes.

In one aspect of the present disclosure, the reaction conditions are inorganic solutions or oil phase, and Pd nanocubes obtained through oilphase synthesis are monodispersed and chemically and physically stablein solution.

In one aspect of the present disclosure, Pd nanocubes having an averageedge length of 3.1 nm were applied as a catalyst in CO₂ reductionreaction and showed excellent selectivity for CO, which can reach amaximum FE of 56% at −1.0 V (FIG. 5). Thus, for example, the presentdisclosure comprises catalytic properties of the metal nanostructuresdisclosed herein, and devices and systems comprising the metalnanostructures enabled herein.

As used herein, the size of a nanocube is defined as the length alongone edge of the cube. If a nanocube has substantial deviations from acube shape, the average length of the edges of the cube can be utilizedto define the size, or, for example, one or more aspect ratios can beused in combination with the length of one edge. As used herein, theterm “nanostructure” refers to a structure having at least one dimensionon the nanoscale, that is, at least one dimension between about 0.1 and1000 nm. It should be understood that “nanostructures” include, but arenot limited to, nanosheets, nanocubes, nanoparticles (e.g., polyhedralnanoparticles), nanospheres, nanowires, nanofibers, and combinationsthereof. A nanocube may comprise a cube having a size on the nanoscale.A nanowire may comprise a wire having a diameter on the nanoscale. Ananoparticle may comprise a particle wherein each spatial dimensionthereof is on the nanoscale.

As used herein, the term “catalyst” refers to a component that directs,provokes, or speeds up a chemical reaction, for example, the reductionof carbon dioxide. Examples of catalysts useful according to the presentdisclosure include, but are not limited to, metal nanocubes, syntheticligands, shape control ligands, and palladium nano structures.

As used herein, the terms “uniform”, “uniform size”, and “uniform shape”are defined as remaining the same in all cases and at all times;unchanging in form or character; provided the same reactants and samereaction conditions, with minimal or defined variation. It should benoted that the methods described herein can provide nanocubes having auniform cube shape, with the aspect ratio of a cube defined as the ratioof the length to the width or the ratio of the length to the height, acube having an aspect ratio of 1, with deviations from cubic shapedemonstrated by an aspect ratio, either length/width or length/height,other than 1. Under the same reaction conditions, the aspect ratio ofthe nanocubes provided by the methods herein can be about 1+/−90%,1+/−80%, 1+/−70%, 1+/−60%, 1+/−50%, 1+/−40%, 1+/−30%, 1+/−20%, 1+/−10%,1+/−5%, 1+/−2.5, or 1+/−1%.

The present disclosure is also directed to palladium nanostructuresprovided by the methods described herein and systems or devicescomprising the palladium nanostructures provided by the methodsdescribed herein, as well as methods of using the same. For example, thedevice may comprise metal nanocubes in a catalyst, the device maycomprise an electrode (such as an electrode for a battery) in a vessel,among other embodiments.

Various aspects of the present disclosure are further described by thefollowing example(s). The word “example” is used herein to mean “servingas an example, instance, or illustration.” Any aspect described hereinas “example” is not necessarily to be construed as limiting, preferred,or advantageous over other aspects.

EXAMPLE 1 Synthesis of Pd-OLA Precursor Stock Solution

Chemicals. Palladium acetylacetonate (99.0%), trioctylphosphine (TOP,90%), oleylamine (OLA, 70%), hexadecylamine (HDA or HAD, 98%),octadecylamine (ODA, 98%), toluene (99.9%), acetone (99%), andchloroform (99.9%), 1-octadecene (ODE, 98%) were purchased fromSigma-Aldrich. Tetradecylamine (TDA, >96%) was purchased from TokyoChemical Industry Co., Ltd. (TCI). Hexane (99%), methanol (99%), andethanol (200 proof) were purchase from Fisher Chemicals. All chemicalswere used as received unless described otherwise.

Procedure: 60 mg of palladium acetylacetonate (0.2 mmol), and 2 mL ofOLA were added into the flask under Ar or N₂ flow. Following Ar or N₂blowing for 20 minutes, the mixed solution was sonicated for 30 min.After sonication, the mixed solution turned clear. The amounts ofpalladium acetylacetonate may vary from 30 mg to 600 mg, and the amountsof OLA and TOP may increase proportionally. The stock solution couldalso be prepared by replacing OLA with TDA, HDA or ODA.

EXAMPLE 2 Synthesis of Pd Nanocubes

6.0 mL of OLA (70%) was loaded in a 25 mL three-neck flask where oxygenwas removed through Ar blowing for 20 min. Subsequently, 1.0 mL (2millimoles) of TOP (at 90% purity) was injected into the flask under Arflow. After 20 min of Ar blowing, the flask was rapidly heated to 200°C. Next, 2 mL of Pd-OLA stock solution (0.2 millimoles Pd) prepared inExample 1 was quickly injected into a hot flask and the reactionsolution turned yellow immediately. The molar ratio of TOP/Pd in thereaction solution was was 10:1. The reaction was held at 200° C. for 20min. Subsequently, the reaction temperature was increased to 250° C.After keeping the reaction solution at 250° C. for another 20 min, thereaction solution was cooled to room temperature. After the temperatureof the reaction solution reached room temperature, 5 mL of ethanol orother hydrophilic solvents, such as methanol or acetone, was injectedinto the reaction mixture. The products were separated by centrifugingat 8000 rpm for 5 min. The supernatant was discarded. A total of 10 mLof hexane and ethanol (v/v, 1:9) was then added to the sediment.Subsequently, the mixture was centrifuged at 8000 rpm for 5 min to washthe products. The washing procedure was repeated twice to removeunreacted precursors and surfactant. The Pd nanocubes were stored inhydrophobic solvents, such as hexane, toluene, chloroform, or mixturesthereof, before characterization.

EXAMPLE 3 Edge Length of Pd Nanocubes

As shown FIGS. 1-3D, the average edge lengths of Pd nanocubes weredetermined by TEM. In one experiment using the present method, uniformPd nanocubes with smaller size were synthesized at 250° C. with areaction time of 20 minutes and with a 10:1 molar ratio of TOP toPd-OLA. The experimental results are shown in FIG. 1. The smaller sizedPd nanocubes was self-assembled and the average edge length of Pdnanocubes was about 3.1 nm.

In another experiment, the molar ratio of TOP/Pd-OLA was kept as 10:1,but the reaction time was prolonged to 60 min. The reaction produced Pdnanocubes having an average edge length of about 5.8 nm. Theexperimental results are shown in FIG. 2.

TOP was chosen to act as the ligand in the present method. The molarratios of TOP to Pd-OLA complex were surprisingly critical to synthesizePd nanocubes. In another experiment, the molar ratio of TOP to Pd-OLAcomplex was adjusted to 15:1. The reaction temperature and reaction timewere maintained at 250° C. and 20 minutes, respectively. The resultingaverage edge length of Pd nanocubes increased to about 5.7 nm. Theexperimental results are shown in FIG. 3A. Under the same reactiontemperature and reaction time, the molar ratios of TOP to Pd-OLA complexwere decreased to 5:1 and 2:1. The 5:1 molar ratio produced Pd nanocubeshaving an average edge length of about 4.3 nm (FIG. 3B) and 3:1 molarratio produced irregular shape Pd nanoparticles with an average size ofabout 1.9 nm (FIG. 3C), respectively. In the absence of TOP ligand, thereaction method produced aggregated Pd nanoparticles (FIG. 3D). Theexperimental results surprisingly showed that the average edge length ofPd nanocubes increased with the amounts of TOP used in the presentmethod and indicated that the TOP molecule not only acted as ashape-control ligand, but also functioned as a reducing agent toincrease the nucleation and growth rate of Pd nanocrystals.

EXAMPLE 4 X-Ray Diffraction, Gas Chromatography, and HPLCCharacterizations of the Products Obtained in Example 2

A Bruker D8 Advance X-ray diffractometer with Cu Kα radiation operatedat a tube voltage of 40 kV and a current of 40 mA was used to obtainX-ray diffraction (XRD) patterns. Transmission electron microscopy (TEM)images were captured using an FEI Tecnai 20 microscope with anaccelerating voltage of 200 kV. Gas products (H₂ and CO) were detectedby gas chromatography instruments (Shimadzu corporation). The separatedgas products were analyzed by a thermal conductivity detector (for H₂)and a flame ionization detector (for CO). Liquid products were analyzedby a high performance liquid chromatography (HPLC, Dionex UltiMate 3000UHPLC+, Thermo Scientific).

EXAMPLE 5 Pd Nanocubes Catalyzed CO₂ Reduction Reaction

Electrochemical CO₂ reduction experiments were conducted using apotentiostat (VersaSTAT MC) in a two compartment electrochemical cellseparated by an anion-exchange membrane (Selemion AMV). A platinum platecounter electrode and a leak-free Ag/AgCl reference electrode(Innovative Instruments, diameter: 2.0 mm) were used in a threeelectrode configuration. A working electrode was prepared bydrop-casting 500 μg of Pd nanocubes, which were dispersed in hexanes,onto a carbon glassy electrode (Catalysts film area: 0.785 cm²). Theworking electrode was dried under argon at room temperature. 2.0 mL ofelectrolyte were added to the working electrode compartment and thecounter electrode compartment, respectively. The working electrodecompartment was sealed in order to allow measurements of gas products.All potentials in this example were converted to the RHE scale by E(vsRHE)=E(vs Ag/AgCl)+0.205 V+0.0591×pH. The 0.1 M KHCO₃ electrolyte wasprepared from K₂CO₃ saturated with CO₂ (pH 7.5).

During the reduction experiment, CO₂ flowed through the workingelectrode compartment at a rate of 5 standard cubic centimeters perminute (SCCM). During chronoamperometry, effluent gas from the cell wasdirected to the sampling loop of a gas chromatography system to analyzethe concentration of gas products. Quantification of the gas productswas performed with the conversion factor derived from the standardcalibration gases. Liquid products were analyzed afterwards by HPLC. Theconcentrations were calculated using calibration curves, which weredeveloped for each individual component. Faradaic efficiencies werecalculated from the amount of charge passed to produce each product,divided by the total charge passed at a specific time or during theoverall run.

To evaluate the catalytic performance of Pd nanocubes, Pd nanocubeshaving an average edge length of about 3.1 nm were used as the CO₂reduction catalysts. The 3.1 nm Pd nanocubes were loaded onto a glassycarbon electrode to serve as a working electrode. FIG. 5 shows both COand H₂ faradic efficiency (FEs) under a variety of applied potentialsvs. RHE. At very negative potentials (−0.9 V vs. RHE), H₂ and CO werethe dominant products. Pd nanocube catalysts showed excellentselectivity for CO, which can reach the maximum FE of 56% at −1.0 V vs.RHE, a performance comparable to commercially available Pd catalysts asreported in the literature. When the potentials reached −1.3 V, H₂became the dominant products for CO₂ reduction reaction. These catalyticresults indicated Pd nanocubes played a very important role to tune theselectivity for the final products of CO₂ reduction reaction.

EXAMPLE 6 Powder X-Ray Diffraction Characterization of Pd Nanocubes

FIG. 4 showed X-ray diffraction (XRD) patterns of Pd nanostructureshaving three different average edge lengths. Pd nanocubes had thestrongest {111} diffraction peak, which was consistent with facecentered cubic (fcc) bulk Pd (Joint Committee on Powder DiffractionStandards, JPCDS 05-0681, XRD peaks are annotated in { }). The reducedsize of Pd nanoparticles led to broadened diffraction peak. With thesize of Pd nanoparticles increasing, the half width of {111} diffractionpeak turned narrower, indicating that the crystallinity of Pdnanoparticles was enhanced with the increased size. Small cubelike Pdnanocrystals were formed in the present method. Although no {200} peakshad been observed, the XRD patterns confirmed the formation of pure Pdcrystals.

While the aspects described herein have been described in conjunctionwith the example aspects outlined above, various alternatives,modifications, variations, improvements, and/or substantial equivalents,whether known or that are or may be presently unforeseen, may becomeapparent to those having at least ordinary skill in the art.Accordingly, the example aspects, as set forth above, are intended to beillustrative, not limiting. Various changes may be made withoutdeparting from the spirit and scope of the disclosure. Therefore, thedisclosure is intended to embrace all known or later-developedalternatives, modifications, variations, improvements, and/orsubstantial equivalents.

Thus, the claims are not intended to be limited to the aspects shownherein, but are to be accorded the full scope consistent with thelanguage of the claims, wherein reference to an element in the singularis not intended to mean “one and only one” unless specifically sostated, but rather “one or more.” All structural and functionalequivalents to the elements of the various aspects described throughoutthis disclosure that are known or later come to be known to those ofordinary skill in the art are expressly incorporated herein by referenceand are intended to be encompassed by the claims. Moreover, nothingdisclosed herein is intended to be dedicated to the public regardless ofwhether such disclosure is explicitly recited in the claims. No claimelement is to be construed as a means plus function unless the elementis expressly recited using the phrase “means for.”

Herein, the recitation of numerical ranges by endpoints (e.g. betweenabout 50:1 and 1:1, between about 100 and 500° C., between about 1minute and 60 minutes) include all numbers subsumed within that range,for example, between about 1 minute and 60 minutes includes 21, 22, 23,and 24 minutes as endpoints within the specified range. Thus, forexample, ranges 22-36, 25-32, 23-29, etc. are also ranges with endpointssubsumed within the range 1-60 depending on the starting materials used,temperature, specific applications, specific embodiments, or limitationsof the claims if needed. The Examples and methods disclosed hereindemonstrate the recited ranges subsume every point within the rangesbecause different synthetic products result from changing one or morereaction parameters. Further, the methods and Examples disclosed hereindescribe various aspects of the disclosed ranges and the effects if theranges are changed individually or in combination with other recitedranges.

Further, the word “example” is used herein to mean “serving as anexample, instance, or illustration.” Any aspect described herein as“example” is not necessarily to be construed as preferred oradvantageous over other aspects. Unless specifically stated otherwise,the term “some” refers to one or more. Combinations such as “at leastone of A, B, or C,” “at least one of A, B, and C,” and “A, B, C, or anycombination thereof” include any combination of A, B, and/or C, and mayinclude multiples of A, multiples of B, or multiples of C. Specifically,combinations such as “at least one of A, B, or C,” “at least one of A,B, and C,” and “A, B, C, or any combination thereof” may be A only, Bonly, C only, A and B, A and C, B and C, or A and B and C, where anysuch combinations may contain one or more member or members of A, B, orC. Nothing disclosed herein is intended to be dedicated to the publicregardless of whether such disclosure is explicitly recited in theclaims.

As used herein, the term “about” and “approximately” are defined tobeing close to as understood by one of ordinary skill in the art. In onenon-limiting embodiment, the term “about” and “approximately” aredefined to be within 10%, preferably within 5%, more preferably within1%, and most preferably within 0.5%.

The examples are put forth so as to provide those of ordinary skill inthe art with a complete disclosure and description of how to make anduse the present invention, and are not intended to limit the scope ofwhat the inventors regard as their invention nor are they intended torepresent that the experiments below are all or the only experimentsperformed. Efforts have been made to ensure accuracy with respect tonumbers used (e.g. amounts, dimensions, etc.) but some experimentalerrors and deviations should be accounted for.

Moreover, all references throughout this application, for example patentdocuments including issued or granted patents or equivalents; patentapplication publications; and non-patent literature documents or othersource material; are hereby incorporated by reference herein in theirentireties, as though individually incorporated by reference.

What is claimed is:
 1. A method for preparing metal nanocubes, themethod comprising: providing a metal complex solution comprising a firstcomplexing agent and a first palladium source selected from palladiumacetylacetonate, palladium acetate, palladium trifluoroacetate,palladium nitrate, and palladium hexafluoroacetylacetonate; heating areaction mixture comprising a shape control ligand under inertatmosphere to a combining temperature; combining the metal complexsolution and the reaction mixture at the combining temperature underinert atmosphere; holding the reaction mixture at the combiningtemperature for a combining time under inert atmosphere; heating thereaction mixture to a reaction temperature under inert atmosphere; andholding the reaction mixture for a reaction time at the reactiontemperature under inert atmosphere to form metal nanocubes.
 2. Themethod of claim 1, further comprising: cooling the reaction mixturecontaining the metal nanocubes; and isolating the metal nanocubes fromthe reaction mixture.
 3. The method of claim 2, further comprisingwashing the nanocubes with an organic solvent.
 4. The method of claim 1,wherein the metal complex solution further comprises platinum, gold,silver, or a combination thereof.
 5. The method of claim 1, wherein themetal complex solution is provided by mixing the first palladium sourcewith the first complexing agent to form the metal complex solution,wherein the first complexing agent comprises an alkylamine selected fromoleylamine, tetradecylamine, hexadecylamine, octadecylamine, andcombinations thereof.
 6. The method of claim 1, wherein the shapecontrol ligand comprises one or more trialkylphosphines.
 7. The methodof claim 6, wherein the shape control ligand is chosen fromtrioctylphosphine, tributylphosphine, and combinations thereof.
 8. Themethod of claim 1, wherein the combining temperature is 185° C. to 220°C. and the reaction temperature is 240° C. to 260° C.
 9. The method ofclaim 8, wherein the combining time is 15 to 25 minutes.
 10. The methodof claim 8, wherein the reaction time is 15 to 25 minutes.
 11. Themethod of claim 1, wherein the metal complex solution is provided bymixing the first palladium source with the first complexing agent untila solution comprising palladium is formed, wherein the first palladiumsource comprises palladium acetylacetonate and the first complexingagent comprises oleylamine.
 12. The method of claim 11, wherein shapecontrol ligand is trioctylphosphine and further comprising wherein themolar ratio of trioctylphosphine to palladium in the reaction mixture isfrom 15:1 to 2:1.
 13. The method of claim 12, wherein the combiningtemperature is 200° C., the combining time is 20 minutes, the reactiontemperature is 250° C., the reaction time is 20 minutes, the molar ratioof trioctylphosphine to palladium in the reaction mixture is 10:1, andthe metal nanocubes comprise palladium nanocubes comprising an averagesize of 3.1 nm.
 14. The method of claim 12, wherein the combiningtemperature is 200° C., the combining time is 20 minutes, the reactiontemperature is 250° C., the reaction time is 60 minutes, the molar ratioof trioctylphosphine to palladium in the reaction mixture is 10:1, andthe metal nanocubes comprise palladium nanocubes comprising an averagesize of 5.8 nm.
 15. The method of claim 12, wherein the combiningtemperature is 200° C., the combining time is 20 minutes, the reactiontemperature is 250° C., the reaction time is 20 minutes, the molar ratioof trioctylphosphine to palladium in the reaction mixture is 15:1, andthe metal nanocubes comprise palladium nanocubes comprising an averagesize of 5.7 nm.
 16. The method of claim 12, wherein the combiningtemperature is 200° C., the combining time is 20 minutes, the reactiontemperature is 250° C., the reaction time is 20 minutes, the molar ratioof trioctylphosphine to palladium in the reaction mixture is 5:1, andthe metal nanocubes comprise palladium nanocubes comprising an averagesize of 4.3 nm.
 17. The method according to claim 1, wherein thereaction temperature is 240° C. to 260° C. and wherein the metalnanocubes have an average size from 1.9 nm to 5.8 nm.